Breaking down the Law-Gospel dialectic

When we say “law” we generally mean “not gospel”; and when we say Gospel, we generally mean “not law”. The two are mutually exclusive categories that don’t share in each other. If this is so, what do we make of Revelation 14:6-7?

Rev 14:6-7 And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people, saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters.

If law is observance of commandments (fear, give glory, worship), and the everlasting gospel teaches us to obey God’s commandments, then are the two really in opposition?